1. Field of the Invention
The present invention relates to a frame structure for a high-temperature denitration apparatus.
2. Description of the Related Art
A conventional denitration apparatus for boiler exhaust gas generally includes, as shown in FIGS. 1 and 2, a box-shaped casing 1 with front and rear open ends for passage of exhaust gas from a boiler (not shown) and a frame body 2 in the casing 1.
The frame body 2 includes a plurality of vertical frame members 3 erected in a spaced-apart relationship widthwise and in a direction of exhaust gas flow of the casing 1, and a plurality of horizontal frame members 4 fixed to the vertical frame members 3 through welding into a latticework. Thus, spaces 5 to be filled with catalysts are formed in a plurality of widthwise rows and in a plurality of vertical stages within the casing 1. Brace members 6 are welded to interconnect slatwise the frame members 3 and 4 for reinforcement against horizontal forces generated, for example, in an earthquake. A plurality of (two in the example shown in FIG. 2) such frame bodies 2 with the same construction are arranged and interconnected in the direction of exhaust gas flow.
The casing 1 is internally lined with thermal insulation material 7, and upon operation, temperature difference is caused between the casing 1 and the frame body 2 so that a degree in thermal expansion of the latter is greater than that of the former. Such difference in thermal expansion between the casing 1 and the frame body 2 upon operation may be absorbed such that the frame body 2 is supported in the casing 1 partly by fixed supports 8 and partly by slide supports 9 so as to make the frame body 2 slidable widthwise and in the direction of exhaust gas flow of the casing 1.
A catalyst port 11 with an openable lid 10 protrudes from a top of the casing 1. With the lid 10 being opened, catalyst units 12 each filled with catalysts are suspended by, for example, cranes as shown in FIG. 2 and are charged into the spaces 5 in the frame bodies 2.
In the above-mentioned denitration apparatus for boiler exhaust gas, the frame members 3 and 4 as components in the frame body 2 are typically made of H-section steel while the brace members 6 are typically round pipes. Superficial dimensions and weights of these members are not the same; typically, the brace members 6 are most lightweight, the horizontal frame members 4 are less lightweight, and the vertical frame members 3 are least lightweight. As a result, as inlet gas temperature rises during activation, the brace members 6, the horizontal frame members 4 and the vertical frame members 3 are increased in temperature in the order named as is clearly shown in FIG. 3. However, since the rising velocity of the inlet gas temperature during activation is not so swift and an activation period is as long as about 2 hours, temperature differences between the members 3, 4 and 6 are not so great and there is no fear of thermal stresses being generated locally between the members during activation, though the members are completely welded together.
Recently, a gas turbine is frequently used for generation of electricity and, in such a case, exhaust gas from the gas turbine must be denitrated. To this end, it has been envisaged that, as a high-temperature denitration apparatus for gas-turbine exhaust gas, the above-mentioned denitration apparatus for boiler exhaust gas is utilized, with no structural change and with material of which the members 3, 4 and 6 are made being changed from carbon steel to stainless steel.
However, in the case of a high-temperature denitration apparatus for gas-turbine exhaust gas, a rising velocity of inlet gas temperature during activation is extremely swift as compared with that in the conventional denitration apparatus for boiler exhaust gas, and the activation period is as short as about 15 minutes; as a result, as the inlet gas temperature rises, great temperature differences are caused between the members 6, 3 and 4 due to differences in superficial dimension and weight between the members as shown in FIG. 4, resulting in a possibility of thermal stresses being generated locally between the members during activation.
During activation of the high-temperature denitration apparatus for gas-turbine exhaust gas, thermal conductivity in the direction of exhaust gas flow between the upstream and downstream vertical frame members 3 is low since it is effected only through the horizontal frame members 4 and the brace member 6 as shown in FIG. 5. As a result, an extremely great temperature difference ΔT (up to about 200° C.) is caused between temperature Tin on a front surface of the upstream vertical frame member 3 and temperature Tout on a rear surface of the downstream vertical frame member 3 so that a great difference in thermal expansion is caused between the upstream and downstream vertical frame members 3, resulting in curved deformation of the frame body 2 backward as shown in FIG. 6, thereby leading to a possibility of excessive thermal stresses being generated. To the contrary, during inactivation, the frame body 2 may be deformed forward, also resulting in a possibility of excessive thermal stresses being generated.
With respect to each of the individual members such as the most upstream frame members 3 and 4, during activation of the high-temperature denitration apparatus for gas-turbine exhaust gas, a great temperature difference Δt is caused between temperatures tin and tout of front and rear surfaces of the frame member 3 or 4 as shown in FIG. 7, resulting in a great difference in thermal expansion between the front and rear surfaces of the frame member 3 or 4, thereby leading to a possibility of great thermal stresses being generated also with respect to each of the individual members.
The invention was made in view of the above and has its object to provide a frame structure for a high-temperature denitration apparatus which can suppress differences in thermal expansion or contraction between the members of the frame body due to temperature differences between them during activation or inactivation, thereby defusing generation of thermal stresses.